1. Field of the Invention
The present Invention relates to a plastic sheet which has creasing lines, each formed of a groove having a bottom surface and a pair of opposed side surfaces each slanting at a certain angle, as well as to a creasing line-forming blade.
2. Description of the Related Art
When a packaging container or casing is formed through bending of a plastic sheet, a groove-like creasing line is press-formed at each portion of the plastic sheet at which the plastic sheet is to be bent. Subsequently, the plastic sheet is bent along the creasing lines in order to complete the packaging container or casing. U.S. Pat. No. 5,302,435 discloses such a technique. Conventional plastic sheets have employed creasing lines of various shapes and structures. The structure of such a creasing line greatly affects the performance of a completed plastic container or casing.
When a packaging container is to be formed through bending a plastic sheet, as shown in FIG. 1, grooves 2 are formed on a sheet 1 punched into a planar shape corresponding to the shape of the container, and the sheet 1 is then bent along the grooves 2 to thereby complete the container as shown in FIG. 2. The grooves 2 are called xe2x80x9clines for foldingxe2x80x9d or simply xe2x80x9ccreasing lines.xe2x80x9d The creasing lines are formed by a process such that a member called a xe2x80x9ccreasing line-forming bladexe2x80x9d is pressed against the sheet 1.
The technique for bending a sheet after formation of creasing lines by use of creasing line-forming blade has conventionally been used for fabrication of paper containers. However, when this technique is applied to fabrication of plastic containers, fabrication of containers having corners of a desired angle is difficult, because a plastic sheet has higher resistance against bending and higher elasticity than does a paper sheet.
Various techniques for solving the above-described problems have been proposed. One solution is employment of creasing lines having a special shape to thereby facilitate bending operation. For example, Japanese Utility Model Publication (kokoku) No. 4-9345 discloses a plastic sheet having creasing lines each formed of a groove in which projections and depressions are formed alternately on the bottom surface along the longitudinal direction thereof. Japanese Patent Application Laid-Open (kokai) No. 64-40317 discloses a plastic sheet having creasing lines each formed of a groove in which holes are formed in the bottom portion at a predetermined pitch along the longitudinal direction thereof.
In relation to a method of bending a plastic sheet, Japanese Patent Application Laid-Open (kokai) No. 2-98422 discloses a method in which a plastic sheet having creasing lines is first folded at each of the creasing lines, then unfolded to the original state, and then subjected to a bending process.
Furthermore, a creasing line-forming blade having an improved structure has been proposed. Japanese Patent Application Laid-Open (kokai) No. 1-141720 (Japanese Patent No. 2541252) discloses a creasing line-forming blade for a plastic sheet whose tip end has depressed portions and projected portions arranged alternately along the longitudinal direction, wherein the depressed portion has a length of 0.3 to 2 mm, the projected portion has a length of 0.02 to 0.15 mm, the tip end of the projected portion has a width not greater than 0.5mm, and the tip angle is 30 to 130xc2x0.
FIG. 3 shows a perspective view of an example of such a creasing line-forming blade. Each projected portion 4 of a creasing line-forming blade 3 has a sharp point, and each depressed portion 5 of the creasing line-forming blade 3 has a flat shape. However, the projected portion 4 is not necessarily required to have the shape of a sharp cutting edge.
FIG. 4 shows a plastic sheet 6 on which creasing lines have been formed by use of the creasing line-forming blade 3. Upon the creasing line-forming blade 3 being pressed onto the plastic sheet 6, the plastic sheet 6 deforms, so that creasing lines 7 are formed. Almost no plastic material remains at each groove portion 8, but plastic material remains at each projection 9 formed by means of the depressed portion 5 of the creasing line-forming blade 3.
A recent automatic container fabrication machine can perform, at high speed, a series of operations for bending a sheet to complete a three-dimensional container, charging a liquid or the like into the container, and sealing the container. Although the above-described techniques have enabled fabrication of containers through bending of a plastic sheet having creasing lines, the conventional plastic sheet cannot completely cope with such an automatic container fabrication machine. Use of the conventional plastic sheet will raise problems such as breakage of a creasing line portion, and failure in formation of a three-dimensional shape.
In a plastic sheet having creasing lines, bending operation is facilitated through a decrease in the thickness (residual thickness) of the plastic sheet at the bottom of each groove serving as a creasing line. However, when the residual thickness is decreased, a strong force tends to act locally at creasing line portions during bending, especially during a step of forming a plastic sheet into a final shape by use of an automatic container fabrication machine, resulting in breakage of the container from a creasing line portion. This problem of breakage becomes remarkable when holes are provided at creasing line portions in order to facilitate a bending operation.
An object of the present invention is to provide a plastic sheet which has creasing lines of improved shape capable of preventing breakage of the creasing lines and which can cope with an automatic container fabrication machine which forms a packaging container from the plastic sheet.
The present invention provides a plastic sheet which has a plurality of creasing lines. Each creasing line comprises a main groove having a narrow bottom surface and a pair of opposed side surfaces each slanting at a certain angle; and a plurality of transverse grooves disposed at a certain pitch in the longitudinal direction of the main groove. Each transverse groove is formed by means of two or more curved projections to have at least one narrow bottom surface extending in a direction substantially perpendicular to the longitudinal direction of the main groove.
The present invention further provides a creasing line-forming blade for plastic sheet. The blade has a narrow tip and extending in the width direction and a pair of opposed side surfaces each continuing from the tip-end surface and slanting at a certain angle. At least two curved depressions are formed at the tip end such that the depressions are located adjacent to each other and extend perpendicular from the tip end. The boundary portion between the curved depressions forms a transverse blade portion.
Since a plastic sheet has high elasticity as compared with a paper sheet, when the plastic sheet is folded along creasing lines which are formed in the same manner as that used with a paper sheet, the plastic sheet restores its original shape due to its high elasticity. Therefore, if such a plastic sheet is formed into the shape of a container, the formed shape may deteriorate with passage of time. Therefore, a manner of forming creasing lines is an important factor in fabrication of containers which do not cause deformation. Furthermore, designing creasing lines in consideration of the material of a plastic sheet to be used is important.
In creasing lines of the present Invention, a plastic material left by means of the transverse blade portion and the curved depressions suppresses elasticity. In addition, a creasing line-forming blade which can cope with any material can be designed through determining the combination of the plurality of curved depressions and longitudinal blade portions.